The present invention relates to a novel fluorescent high-pressure mercury-vapor lamp (to be referred as "mercury-vapor lamp" in this specification) which provides excellent color rendition and whose color temperature may be arbitrarily varied. More particularly the invention relates to the composition of the phosphor coating applied over the inner surface of the outer tube of a mercury-vapor lamp.
With the conventional mercury-vapor lamps for exterior lighting consisting of an arc tube and a transparent outer tube, the yellow-green light emitted is dominant and the lighting appears unnatural. The yellow-green light is dominant since the spectral energy distribution of the mercury emission does not extend over the wavelength of 579 nm. In addition a discontinuity in the mercury emission spectrum exists between 436 and 546 nm; that is, in the blue-green region. The defect, due to the former reason, has been remedied by the use of a phosphor coating applied over the inner surface of the outer tube of the mercury-vapor lamp which is capable of converting the mercury emission into a red light emission. The red light emitting phosphors used for this purpose are, for instance, YVO.sub.4 : Eu, YV.sub.0.5 P.sub.0.5 O.sub.4 :Eu. However, a discontinuity still exists between 436 and 546 nm; that is, light produced by the mercury-vapor lamp lacks blue-green light. The red light is dominant so that satisfactory interior lighting with a desired color rendition cannot be attained.